The present standard, A.A.R. approved, ABDW Freight Brake Control Valve manufactured by Westinghouse Air Brake Company includes an accelerated application valve portion that is mounted on the valve emergency portion and operates in response to the quick-action chamber "breathing" pressure developed by the emergency piston during service brake applications. Quick-action chamber "breathing" is necessary during service brake applications in order to stabilize the emergency piston against undesired emergency operation. The exhaust backpressure of this quick-action chamber "breathing" pressure is utilized to pilot the accelerated application valve which, in turn, operates to effect a local, quick service reduction of brake pipe pressure in a cyclic manner during service brake applications to supplement the trainline reduction of brake pipe pressure at each car of the train, thereby hastening propagation of the brake pipe signal through each car of a train.
It will be appreciated, therefore, that during a service brake pipe reduction, operation of the accelerated application valve is predicated on movement of the emergency piston to a position in which the quick-action chamber pressure on one side of the piston is vented to atmosphere, and thus reduced at a rate corresponding to the existing service rate of reduction of the brake pipe pressure effective on the other side of the piston, thereby preventing the emergency piston from being moved further to an emergency application position.
It will be further appreciated that this emergency piston includes a slide valve via which the quick-action chamber pressure venting or "breathing" connection is established. In addition to providing the quick-action chamber "breathing" function, the emergency piston slide valve also provides for piloting the vent valve to obtain quick-action emergency. It is well known that slide valve construction, while having a long maintenance-free service life, is somewhat more expensive than other valve-type constructions. However, multiple valve control functions can be incorporated in a relatively small valve device to offset the initial expense of construction.
In U.S. Pat. No. 4,690,443, there is disclosed an arrangement for incorporating the accelerated application valve function in the emergency piston slide valve. It will be appreciated, however, that one characteristic of slide valve operation is that the first function to occur in one direction of movement is the last function to occur during movement in the opposite direction. Therefore, in order that the accelerated application venting of brake pipe pressure be terminated prior to terminating the venting of quick-action chamber pressure to assure that the accelerated application is not perpetuated, the accelerated application venting of brake pipe pressure is initiated after the venting of quick-action chamber pressure. Accordingly, the accelerated application function is not positive in its operation, since relatively light brake pipe pressure reductions may be counteracted by the venting of quick-action chamber pressure before the emergency piston has moved sufficiently to effect the accelerated application function.
Another emergency piston slide valve arrangement is disclosed in U.S. Pat. No. 4,744,609 in which two slide valves are employed to provide positive cut-off of the accelerated application function during a service lap condition, without sacrificing positive actuation of the emergency piston to accelerated application position during brake application.